Question: Ingrid hit a golf ball. The height of the ball (in meters above the ground) $t$ seconds after being hit is modeled by $h(t)=-5t^2+30t$ Ingrid wants to know when the ball reached its highest point. 1) Rewrite the function in a different form (factored or vertex) where the answer appears as a number in the equation. $h(t)=$ 2) How many seconds after being hit does the ball reach its highest point?
Choosing a form The ball's highest point relates to the maximum of the function. Which form reveals this feature? Here's a summary of what each form reveals along with examples. Note that these are all equivalent forms of the same function, but not the function modeling the height of the ball. Form Example Feature revealed Standard $f(x)=2x^2-12x+{10}$ $y$ -intercept is ${10}$ Factored $f(x)=2(x-C{1})(x-C{5})$ Zeros are $x=C1$ and $x=C5$ Vertex $f(x)=2(x-{3})^2{-8}$ Vertex is $(3,{-8})$ Rewrite in vertex form The vertex of the function tells us the value of $t$ where the function reaches its maximum height, so let's rewrite $h(t)$ in vertex form by completing the square. The number that will help us complete the square is $\left(\dfrac{{-6}}{2}\right)^2={9}$ : $\begin{aligned} h(t)&=-5t^2+30t \\\\ &=-5(t^2-6t)&&\text{Factor } -5 \text{ from first two terms}. \\\\ &=-5(t^2{-6}t+{9}){+45}&&\text{Complete the square}. \\\\ &=-5(t-3)^2+45&&\text{Factor}. \end{aligned}$ [How do we know what to add to complete the square?] When does the ball reach its highest point? The vertex form of the function reveals its vertex, and we know this point is a maximum for $h(t)$ since the leading coefficient $-5$ is negative. In general, for any quadratic function written in vertex form $f(x)=a(x-{h})^2+{k}$, we can conclude that the vertex is the point $({h},{k})$. So for $h(t)=-5(t-{3})^2+{45}$, the vertex is $({3},{45})$, and we know the ball reaches its highest point at $t={3}$ seconds. Answers 1) The vertex form of the function reveals when the ball reaches its highest point: $h(t)=-5\left(t-3\right)^2+45$ 2) The ball reaches its highest point $3$ seconds after being hit.